I doubt these are very common with a frame long enough for a 13' flatbed. That length frame, if you find it, only comes with a regular cab truck. What is mistly available is a 60" or 84" CA frame alowing for a 9' or 11' flatbed respectively.
An 11'bed automatically solves yoyr rear jack problem as they will be past the end of the bed. Now you just need to deal with the drain pipe. When I was considering doing this there were a few campers that were flat on the bittom all the back but I firget whuch ones.
If you did use a regular cab truck and 13' bed yoy might actually overload the front axle by getting the camper COG too far forward.